1. Field of the Invention
The present invention relates to a method of manufacturing a crystalline semiconductor thin film, and more particularly, to a method of forming a low-concentration crystalline semiconductor thin film on an amorphous substrate or a poly-crystalline substrate.
2. Description of the Related Art
In general, a MOSFET adapted to a switching device includes a channel region which is converted into a conductor by controlling a gate, high-concentration electrodes (drain and source electrodes) at two sides of the switching device, a gate oxide layer, and the gate, which are formed by performing an implant process and other general semiconductor processes. The channel region is made of a low-concentration semiconductor thin film having an impurity concentration of about 1019/Cm3 or less. The low-concentration semiconductor thin film used for forming the channel region becomes an important factor for determining switching characteristics and needs to be crystallized.
As conventional low-concentration semiconductor thin films that are made of silicon (Si), a hydrogenated amorphous silicon thin film (a-Si:H), a microcrystal silicon thin film (uc-Si), and a crystal-grown silicon thin film are mainly used.
The hydrogenated amorphous silicon thin film or the microcrystal silicon thin film (uc-Si) that are used for manufacturing a large-sized LCD has an advantage in that the thin films can be manufactured with a simple manufacturing method. However, since many defects are included in the thin film, a mobility of charges is low and a life cycle is short, so that switching characteristics of the switching device are not good. On the other hand, the crystal-grown silicon thin film that is used for manufacturing a small-sized high-image-quality LCD has an advantage in that the switching characteristics of the switching device are good due to a good mobility of charges and a long life cycle. However, the thin film has disadvantages in that the process of crystallizing silicon is complicated and a long time is taken to manufacture the crystal-grown silicon thin film.
As an example of taking all the advantages of these methods, there have been proposed methods of crystallizing a low-concentration semiconductor thin film. Various methods of crystallizing the low-concentration semiconductor thin film have been researched. As the most general method, there is a method of depositing an amorphous semiconductor thin film and performing thermal treatment. However, the method has a disadvantage in that a time for the crystallization process is too long. For example, a process of crystallizing a silicon thin film deposited on a Corning glass requires for the thermal treatment at a temperature of about 700□ for 4 or more hours. Therefore, a productivity of the low-concentration crystalline silicon thin film is very lowered.
As another example, there is an MIC (metal induced crystallization) method. In the MIC method, silicon is deposited on a thin metal layer made of aluminum (Al), nickel (Ni), or the like, and after that, thermal treatment is performed at a temperature of 450□ or less, so that the metal elements and the silicon are dislocated and the silicon is crystallized. Since the method utilizes low-temperature thermal treatment, the method has an advantage in that limitation to a material for an underlying substrate is alleviated. However, since many metal impurities are included in the crystallized silicon thin film, the method has a disadvantage in that characteristics of devices deteriorate.
As still another example, there is a method of crystallizing an amorphous silicon layer through an induction heating process.
FIG. 1 illustrates a general method for manufacturing a crystalline semiconductor thin film through an induction heating process. Now, the method of crystallizing an amorphous silicon layer through the induction heating process will be described in brief with reference to FIG. 1.
Firstly, a diffusion barrier 120 and an amorphous silicon layer 130 are formed on an upper portion of a substrate 110. The amorphous silicon 130 is disposed under an induction coil 152. Next, an alternating current generated by a current generator 151 is applied to flow through the induction coil 152, so that an alternating magnetic field affects the amorphous silicon layer 130.
Carriers in a portion (b) of the amorphous silicon layer 130 under the induction coil are rotated by the alternating magnetic field, so that Ohmic Heating occurs. Therefore, the portion (b) is changed into a fluid state, and the fluid-state portion is crystallized in an interface (d) between the portion (b) and solid-state portions (a) and (c) by using the solid-state portions (a) and (c) as a seed. Next, the amorphous silicon layer 130 is moved in a specific direction (e), and thus, the other portion (c) is crystallized.
Since the heat generated in the inductively heated portion depends on the number of carriers such as free electrons, the method can be adapted to a silicon thin film having a high impurity concentration. However, since a low-concentration semiconductor thin film used for manufacturing a switching device has a relatively small number of carriers, the method has a disadvantage in that the method cannot be adapted to the low-concentration semiconductor thin film. Therefore, in order to form a low-concentration crystalline semiconductor thin film by using the method, a separate process of reducing the impurity concentration is needed after the high-concentration semiconductor thin film is crystallized.
In order to crystallize the low-concentration semiconductor thin film through the induction heating, there is needed a pre-heating process for increasing a temperature of the low-concentration semiconductor thin film up to a high temperature so as to generate a sufficient number of thermal electrons. The generated thermal electrons are used as carriers for the induction heating. However, since the temperature of the pre-heating is increased as the impurity concentration of the semiconductor thin film is decreased. Therefore, there is a limitation to a type of material for the underlying substrate used for the serialization of the low-concentration semiconductor thin film.